172 GREEN PLANTS AS FOOD MAKERS 



a new inner layer of (9). The breathing holes on the sur- 

 face of bark are called (10). The main bulk of a monocoty- 



ledonous stem is made up of (11), through which (12) 



(13) are scattered. The rise of water in a stem is brought 



about by the (14) of water from the tree and by the 



(15) of (16). Foods are (17) and are 



transported to all parts of the (18). 



cork 



outer corteK 

 vascular 



--pith. 



-bud. 



PROBLEM X. WHY ARE PLANTS MODIFIED? 



Modified stems. We have already seen that the factors of the 

 environment, light, heat, gravity, moisture, air currents, and other 



factors act upon the living 

 substance of plants, caus- 

 ing them to react in vari- 

 ous ways. The changes 

 which take place usually 

 fit the plant to succeed 

 better in its battle for life. 

 Thus various modifications 

 of stems have been brought 

 about. The potato tuber 

 is simply a much thickened 

 storage stem. The tiny 

 projection growing within 

 the eye is a bud, which 

 may give rise to a branch 

 later. Some stems have 

 come to exist underground 

 because of the protection 

 thus afforded. The pest 

 called couch grass or quick 

 grass has such a stem. 

 Bulbs, like the onion or 

 lily, are examples of stems which have become shortened and 

 covered with thickened leaves, filled with food. Still other stems, 

 like that of the dandelion, have become reduced in length, which 

 prevents them from being broken off by grazing animals. 



scaler 



-/-terminal KtcC 

 lateral "bucC 



Cross sections of a potato and of an onion. How 

 can you show that these are modified stems? 



